Popular auto white balance control used in recent video cameras and the like is executed not using any external sensor but using an output from an image capturing element. As an example of white balance control of this type, color difference signals (R-Y and B-Y) are obtained from a luminance signal (Y) and red (R), green (G), and blue (B) color signals. Image data is divided into small blocks, signals are averaged in each divided block, and color signal components of almost white are extracted from the averages. The white balance is so controlled as to adjust the average of the extracted color signals of almost to be equal to a target white value (e.g., Japanese Patent Laid-Open No. 5-64219).
In white balance control of this type, it is known to presume a light source on the basis of illuminance information of an object. Generally, the illuminance of an object tends to represent a relatively large numerical value under outdoor sunlight and a relatively small numerical value under an indoor artificial light. From this, it can be presumed from the illuminance of an object to a certain degree whether the light source is outdoor sunlight or an indoor artificial light such as a fluorescent lamp. It is presumed that the light source is an outdoor one when the illuminance of an object is higher than a predetermined threshold and an indoor one when the illuminance is smaller. The white balance gain is determined in accordance with the light source information (indoor/outdoor).
FIGS. 17A and 17B are graphs each showing a white extraction range corresponding to the type of light source. In FIGS. 17A and 17B, reference numeral 1700 denotes a white extraction range on the color difference plane. A luminance signal which is larger than a predetermined threshold and falls within the white extraction range is extracted. The white extraction range is corrected so that a blue-based color is determined to be achromatic, as shown in FIG. 17A, when light source information represents an outdoor light source, and a red-based color corresponding to a halogen lamp and a green-based color corresponding to an artificial light are determined to be achromatic, as shown in FIG. 17B, when light source information represents an indoor light source. White data is extracted using the white extraction range corresponding to the type of light source. The white balance gains of red (R) and blue (B) signals are so determined as to adjust the extracted white data to the target white value. Then, white balance control is executed.
As described above, whether the light source is an outdoor or indoor one is presumed on the basis of illuminance information. In this case, when the object is photographed at a dark outdoor place, e.g., in the shade, the scene may be misjudged as an indoor one. This is because the distributions of outdoor light and indoor light partially overlap each other in a threshold process based on only the illuminance and light sources cannot be discriminated by only the threshold of a linear characteristic.
When green vegetation or a green lawn is photographed in the outdoor shade, the white extraction range contains the green region, and the function of changing green to white (achromatic color) acts. If a picture is taken in this state, the object which is originally white is photographed in magenta, and green of vegetation and a lawn changes to gray, failing in proper white balance control.